It is very important to develop a simple, safe, and reliable system for coordinating cancer brain surgery while it is actually being done. This is true for simple stereotactic techniques and also true for resections. Currently there are a number of devices that allow integration of pre- operative stereotactic coordinates to intra-operative use. This allows tumor localization and a high degree of accuracy, at least at the beginning of surgery. The major problem is that as the surgery proceeds, and as the brain either relaxes or swells, and as some tumor is removed, the coordinates change. In this regard, it would be nice to have ultrasound guidance during the operation which was initially coordinated with stereotactic information derived from 3-D stereo computer reconstruction with prior MRI or CT scans in stereotactic space. But, in fact, the use of ultrasound is probably the only way of carrying out this real-time, on-line guidance, since the use of both CT and MRI for this purpose would be unfeasible froth both a physical and financial viewpoint. In addition, CT would involve x-ray exposures, and MRI presents a strong magnetic field and would require the use of only non- magnetic metallic instrumentation. On the other hand, the ultrasound probe can easily be placed on the brain, either through an adjacent hole in the skull or through the surgical brain access.